This invention relates to a hollow reactor, permitting the growth of sessile microorganisms, for biological waste water purification.
Various media configurations, designed for the conditions of the particular processing technology such as trickling filters, immersed reactors or fixed beds in activated-sludge plants, are used for growing sessile micro-organisms in biological waste-water purification plants. The media configurations in immersed-reactor plants have to perform an extensive function, since not only must they provide the largest possible growth surfaces while preventing clogging of the cavities, but also they have to assure the oxygen and nutrient transport to the fixed biomass as well as sufficient circulation of the waste water. Simple immersed-reactor systems can satisfy these requirements only approximately with very low loading capabilities, meaning a large and expensive plant volume, whereas systems with better output capabilities necessitate a greater expenditure on technical equipment as well as greater power consumption.
The influence of the sessile microorganisms on the particular purification process is characterized by the ratio of volume of medium to waste-water volume contained in the reactor:
______________________________________ Trickling filters ca. 100:1 Immersed-reactor systems 0.8-2.0:1 Activation-with fixed beds ca. 0.2:1 ______________________________________
The rotationally symmetric mechanisms used in the immersed-reactor process must assure within the system the supply of oxygen and nutrients as well as adequate mixing. In simple form, disks mounted in parallel on a shaft and immersed in waste water to at most the hub center rotate in a semicylindrical trough. To improve the oxygen supply and waste-water mixing, immersed reactors are known whose growth media consist of spiral or helical coils. the helical immersed reactors include the immersed reactors known by the designations of immersed coil and biospiral. In the case of the immersed coil, tubes are coiled in layers onto a shaft in the longitudinal direction, so that there is always a constant distance between the axis of a given tube and the drum axis. In the case of the biospiral, a surface resembling a horizontal worm conveyor with large outside diameter and small pitch is attached helically to a shaft. If the helical immersed reactors, whose central axis is mounted in the vicinity of the water level line, are made to rotate, slight transport of the waste water, corresponding to the low speed of rotation, takes place in the direction of the longitudinal axis.
The disk-type and helical immersed reactors operate without pressure, and only around half of their total surface area is in contact with the waste water. In one immersed reactor which has been disclosed as a coiled reactor, both a trapezoidal film and smooth film are coiled together on a shaft, thus forming a spiral reactor with lateral holes. During rotation of the immersed reactor, whose axis is also located in the vicinity of the water level line, the waste water on emergence drains to one side and on immersion forces the air to the other side, because of the trapezoidal channels running helically with respect to the central axis. For the part of the coiled reactor below the water level line, only a minor through flow is to be expected, and this is brought to a stop by fairly intensive biological growth.
A hollow reactor formed from two tubular spirals coiled in opposite directions has been disclosed by U.S. Pat. No. 4,351,721. The characterizing features presented are that the two inner tube ends of the double spiral are connected together, and the axis of the spiral-tube reactor is mounted so far above the water level that the outer tube end of one spiral pointing against the direction of rotation dips into the waste water and scoops in a small volume of waste water. During further rotation, the scooped-in volume of waste water rises through the spiral and passes via a horizontal connecting piece into the opposite tubular spiral, so that it is transported down again. The alleged advantage of the double spiral immersed only slightly in the waste water is expressly stated: constant alternating flushing and aeration of the waste water inside the tubes, as well as significant energy savings due to torque compensation by virtue of the opposite directions of the interconnected double spiral. In contrast to the good aeration of the waste water inside the tube, there is the disadvantage that only an extremely small volume of waste water can be brought into contact with the biological growth for a relatively short time. The outside surfaces of the tubular spiral, except for the outer spiral turn, do not come into contact with waste water at all. Thus the technical designation of "immersed reactor" cannot even be applied to this device (see ATV (allgemeine technische Vorschriften) sheet A 135). The major effect of the torque relief achieved by the opposing coupled tubular spirals is merely an energy savings. Since the major part of the apparatus is outside the water, enormous bearing pressures occur, resulting in a greater power requirement for the drive of the device. The specific power consumption, expressed in W/m.sup.2 of installed area of the medium, is 0.92 W/m.sup.2 on the basis of the information presented.
An immersed reactor, which is largely immersed in the waste water and thus creates favorable prerequisites for the largest possible area of contact between fixed biomass and surrounding waste water, has been disclosed in West German Offenlegungsschrift No. 33 24 853. The immersed reactor, which essentially consists of a perforated drum in which the packing material is contained, is shaped on its outside such that, at sufficient speed of rotation, air bubbles are entrained into the waste-water tank and an oxygen-rich waste water is produced in the vicinity of the immersed reactor. This apparatus is therefore primarily an aerating system with a immersed-reactor component connected in series. Despite the high speed of drum revolution needed to entrain oxygen, controlling waste water exchange in the packed inside of the drum is not assured.
Cellular-wheel immersed reactors with segment-like pockets have the property of taking up surrounding air and releasing it again at a specified position under water. These cellular wheels operate on the reverse of the bucket-wheel principle. Since the air trapped in the pockets is released only below the wheel axis, even reactor surfaces which do not emerge from the waste water can be supplied with oxygen. In this design approximately 2/3 of the diameter of the immersed reactor projects into the waste water. Another design has a single pocket whose circumference is trumpet-shaped and which, on emergence, takes up a large air volume and as rotation continues compresses the air so strongly that the air at the other end already escapes when this end is at its lowest point. To be able to generate this high pressure, however, only half the wheel must be immersed in the waste water. The drive of such immersed reactors which trap and release air below the water line requires a relatively high power consumption, since the trapped air is kept at the periphery of the wheel for 1/4 of one revolution.
The structural design of cellular wheels and the surface shaping of immersed disks to increase the area available to biological growth result in high manufacturing costs and expensive assembly operations. From the viewpoint of process engineering, the known immersed reactors suffer from the defect of nonuniform surface coverage (sludge load per unit area per unit time), since no measurable waste-water exchange occurs within the tank housing the immersed reactor and in the cavities and interstices of the immersed reactors themselves, and since the flow to the reactor surfaces covered with microorganisms is very variable.